1. Field of the Invention
The present invention relates generally to apparatus and method for communications and more specifically to mobile radio communications systems.
2. Description of the Related Art
Control channels are used in cellular systems to setup and control (hence the name) outgoing call origination from and incoming call reception by mobile stations. In traditional public cellular systems a minimum number of channels in the cellular spectrum is dedicated to and reserved for use as control channels. Private cellular systems that use portions of the overall public cellular system are prohibited from utilizing the public system's control channels as conflicts among public and private users may occur.
In public cellular systems such as Advanced Mobile Phone Service (AMPS), 21 dedicated control channels exist for each cell and are fixed in frequency systemwide. The remaining channels may be used for voice/data activity. When a mobile station scans the frequency spectrum in search of a control channel, at most 21 predefined frequencies need be scanned. This amount of scanning results in no appreciable delay in operations and is acceptable. However, this is not the situation with private cellular systems.
Private cellular systems operate within and use the public cellular system spectrum and are required by law to be transparent to other users of the public system. A typical private system uses a number of voice/data channels from the public cellular system. This number of public system voice/data channels includes both voice/data and control channels for the private system. As private control channels operate on public voice/data channels and are assigned to frequencies on a dynamic and somewhat arbitrary basis depending on current traffic needs and load of the underlying public systems, reserving a fixed control channel may not be practicable.
The cellular industry is venturing into new markets of micro-cellular applications such as wireless PBX and home residential wireless systems. This has been most evident by the recent release of the IS-94 standard, and subsequently, IS-91-A, designed for cellular microcell coverage and operations, and is especially suitable for private systems that support closed user group(s) and home residential systems in the 800 MHz public cellular systems. Since one of the mandatory requirements for private cellular systems is that public service not be impacted, non-public microcells must be totally transparent to terminals operating in the public cellular network. This requirement prohibits private microcells from using the standard control channels (i.e., the aforementioned 21 channels as specified in the AMPS/TDMA/CDMA standards). Consequently, private microsystems have to "steal" the unused cellular voice/traffic channels as their control channels. As such, the location of control channel for a private cell may be determined on a dynamic basis, depending on the actual call activities and frequency planning of the under-laying macro cell. The exact location of a control channel for a private system can be anywhere in the spectrum other than those reserved for the 21 control channels.
The somewhat arbitrary location of control channels of private systems places significant burden on the mobile stations to quickly lock onto the control channels to obtain service for the following reasons. First, the mobile terminals have to scan a significantly larger number of channels to find a control channel. For example, IS-94/IS-91-A specifies a channel block with a maximum of 128 channels for mobiles to scan for a control channel. Second, since this block of channels is actually the voice/traffic channel block of the under-laying macro cell, the mobile station must differentiate between control channels and voice/traffic channels. Third, because private systems only intend to provide services to closed user group(s), not every microcell will grant service to a particular mobile. Mobile stations may exhaust all existing micro control channels in places where multiple control channels exist due to overlapping microsystems to lock onto a system that grants their service.
One method for the purpose of finding non-standard control channels from a block of voice/traffic channels of the under-laying macrocell is based on achieving word-sync on each channel by the mobile station. In IS-94/IS-91-A, it is specified that the mobile station must determine whether word-sync can be achieved for each channel in the channel block.
A drawback of this scheme is that to determine if word-sync is achievable, the mobile station has to wait as long as 46.3 msec per scanned channel for the synchronization word for every channel in the block whose Radio Signal Strength reaches above a minimum threshold. Considering that there are as many as 128 channels in a block, this leads to unacceptable scanning delays of up to 5.93 seconds.
There is accordingly a need for a new method to expediting the search for control channels of private systems to reduce scanning time in order to solve or ameliorate one or more of the above-described problems.